1. Field of the Disclosure
The disclosure relates to a separation method for suspended solids. The method can be applied to a gas flotation separator that does not include a mechanical means (e.g., rakes, skimmers) for removing a float layer of aerated solids. The method employs a control process using the instantaneous height of fluid in the separator to control the effluent flow rate of clarified liquid from the separator to achieve a steady and continuous operation of the separation process.
The separation method can generally be used to treat wastewater. For example, the separation method can be used in a solids and nutrient recovery system for applications that require partitioning of the solid and liquid phases of effluents from food processing residuals and waste, residuals from animal feeding operations, and the effluent from the digestion of these residuals. Specific applications include the separation of suspended solids from the effluent of anaerobic digesters where the separation may be driven by ambient air (or other gas, for example biogas) or where the suspended solids are of a fragile or delicate nature that would otherwise be disturbed by mechanical removal methods. The recovered suspended solid emulsion may be recycled as a feed substrate to digesters for the generation of biogas for power generation.
2. Brief Description of Related Technology
The separation of solids and liquids is prevalent in wastewater treatment. Generally the separation involves a first mechanical step (e.g., filtering of settleable solids) and then proceeds to a more aggressive phase separation prior to final treatment. Thus, after some type of mechanical separation, the wastewater treatment generally includes the recovery of suspended solids or emulsions that exist in the particle range between settable solids and dissolved solids. In a dissolved gas floatation device (generically a “DAF” device, even when the dissolved gas is other than air), the solid/liquid mixture to be separated is treated with chemicals, and then fine bubbles in the range of 10 to 20 microns are introduced to the mixture. The solid/liquid mixture is then directed to an open separation tank for buoyant separation. A float layer of aerated solids rises to the top of the separation tank and is skimmed from the top of the separation tank contents with a mechanical rake (or other mechanical collection means). The result is a clear or generally clear effluent and a concentrated solids emulsion.
Separation using principles of buoyancy (e.g., flotation separation) is advantageous because it achieves high capture rates while producing a clean effluent. Flotation separation can also concentrate or recycle the waste solids. Concentrated waste streams are desirable to minimize the size of downstream processing facilities. Maximizing the float solids concentration is advantageous since the solids concentration affects downstream processing resources and cost. If the solids produced are dilute, the downstream dewatering or disposal costs increase. If the separator is used in a biological process incorporating solids recycled from the separator, the processing cost and reactor size are greater if dilute solids are produced.
Flotation separation can be used for both clarification and thickening. Flotation separation can remove suspended solids, colloids, and oil and grease at the same time. Flocculation and coagulation agents can be added to the flotation stream to consolidate solids and remove nutrients, as well as bacteria and other organisms. Flotation separation takes advantage of the hydrophobic interactions that are lacking in other separation technologies.
The use of mechanical float harvesting methods and/or devices is often unsuitable for the separation of delicate or fragile emulsions (e.g., since the harvesting means can destroy the emulsion) or when it is desired to maintain anaerobic conditions of the emulsion (e.g., since mechanical harvesting systems are generally open to the ambient environment), for example when the separated solids are to be recycled to an anaerobic digester for further use and processing.
U.S. Patent Publication No. 2008/0190859, the contents of which are incorporated herein in their entirety, describes a DAF separator without mechanical float harvesting means. The disclosed cylindrical separator includes tangential feed inlets to generate a swirling flow and a corresponding centrifugal force that drives a float layer of aerated solids radially inward toward a central weir for collection of the concentrated solid effluent.
Objects
Solid/liquid separation processes are desirably operated in a continuous mode (i.e., as opposed to a batch or mode). However, the foregoing processes are difficult to operate continuously because many normal process variations (e.g., inlet flow rate; inlet solids type/content, charge, and density; aerating gas feed rate; coagulation and flocculation agent feed rates) cause the resulting conditions within a separator to be unsteady. The desired distribution between aerated solids (e.g., in an upper float layer) and clarified liquid (e.g., in a lower clarified layer) present in the separator can vary with time. Failure to accurately control the distribution between the two separation phases can undesirably lead to aerated solids exiting through the clarified liquid effluent and/or clarified liquid exiting via the aerated solids outlet.
Such problems can be addressed by operating the separator in a batch or semi-batch mode. For example, an operator can intermittently initiate a clean cycle in which the inlet flow to the separator is increased to flush accumulating aerated float solids through the solids outlet. However, the clean cycle interrupts the efficient continuous operation of the separator, because a substantial portion of the inlet cleaning fluid exits with the flushed solids, thereby undesirably diluting the aerated float solids. Attempts at controlling the separation process (e.g., by attempting to balance inlet and clarified effluent flow rates in the DAF separator of U.S. Patent Publication No. 2008/0190859) have met limited success, resulting in an unsteady distribution between aerated solids and clarified liquid, thereby still requiring frequent cleaning cycles (e.g., 15-minute cleaning cycles approximately every 1-2 hours).
There is a need for solid/liquid separation processes (e.g., DAF separation processes) that can be reliably operated in a continuous mode with minimal or no process interruptions (e.g., by minimizing or eliminating the need for intermittent cleaning cycles). As a complement to this, there is also a need for control processes that permit accurate control of the distribution between aerated float solids and clarified liquid present in the separator. These and other objects may become increasingly apparent by reference to the following description and drawings.